Hi Eileen
Muster rolls are the usual way to follow a man's army career. You would have to go to TNA at Kew. Records for the 55th are in WO12.
For the years 1817-18 the reference is WO 12/6486.
There may have been more than one John Evans in the 55th, but yours was a corporal for the last nine years of his fourteen, which should help.
Apparently there are some muster rolls for the 55th, 1815-1816, online on ancestry in British Army Muster Books and Pay Lists, 1812-1817
If you are in doubt about the Jersey baptisms (and I'm not sure that you are!), you could investigate the army births a bit more.
We have
Jane Evans
place Jersey
year 1818
regiment 55th
vol 1088 page 15
If you were to apply to the GRO for this, what exactly would you get? I've absolutely no idea, perhaps Max could help.
John